In operation of a high intensity discharge lamp, visible radiation is emitted by the fill at relatively high pressure upon excitation typically caused by passage of current therethrough. One class of high intensity discharge lamps comprises electrodeless lamps which generate an arc discharge by establishing a solenoidal electric field in the high-pressure gaseous lamp fill comprising the combination of one or more metal halides and an inert buffer gas. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an induction coil surrounding an arc tube which contains the fill. The arc tube and induction coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the induction coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the induction coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, producing a toroidal arc discharge in the arc tube.
A life-limiting phenomena in an electrodeless HID lamp is damage to the arc tube wall, especially the portion nearest the plasma arc discharge where the capacitively coupled electric field between the arc and the coil is the highest. Much of this damage may be attributed to filamentary discharges which form in the capacitive field between the induction coil and the plasma arc discharge.
Accordingly, it is desirable to reduce the electric field strength between the induction coil and the plasma arc discharge, thereby extending the useful life of the lamp and improving the luminous output therefrom.